Square, the payments company known for its smartphone and tablet credit-card readers, will offer its checkout system at several Whole Foods stores, making the grocer the second national retailer to allow card processing from the startup.

The deal comes more than a year after Square began processing payments at thousands of Starbucks outlets and suggests the San Francisco company is looking to expand more aggressively beyond the food trucks and mom-and-pop shops where it is commonplace.

Square will enable checkout at sites within Whole Foods stores, such as fresh sandwich and coffee stations, though not at the main registers. Whole Foods has installed Square checkout stands at seven U.S. stores and will add locations in coming months, a Square spokesman said.

“This is an important move for us, it gives us the ability to scale,” said the spokesman, Aaron Zamost. He declined to discuss terms of the deal with Whole Foods, including at what price the retailer will buy the checkout stands, which are designed to hold tablet computers.

A bevy of startups is jostling for position in the digital-payments industry, today dominated by eBay’s PayPal. Square, which takes a 2.75% fee from each transaction, in 2012 raised money at a $3.25 billion valuation; another startup, Stripe, recently landed a $1.75 billion valuation. PayPal last year acquired Braintree Payment Solutions for $800 million.

Having another well-known retailer as a partner could bolster investor faith in Square as it moves toward an initial public offering potentially as soon as this year. The company, founded and run by Twitter co-founder Jack Dorsey, held preliminary talks with at least two banks about a possible IPO in 2014, a person familiar with the matter said in November.

Dorsey has said the company isn’t yet profitable.

For Whole Foods, based in Austin, Tex., installing multiple registers within its grocery stores could help speed lines and attract new customers, such as those who use Square’s app that allows for checkout through a smartphone without swiping a credit card.

“We’re looking at new ways to offer seamless technology that will respond to customers’ needs,” Whole Foods CEO Walter Robb said in emailed comments.

It’s not clear if Whole Foods plans to use Square for customers checking out with every day groceries. Square today doesn’t offer scales for weighing produce, among other limitations.

A Whole Foods spokeswoman declined to comment on the deal’s specifics.

As well, Square may face a fierce new competitor in in-store checkout. Amazon.com is preparing to offer Kindle tablets to retailers for credit-card payment processing as soon as this summer, people familiar with the plans told The Wall Street Journal last month.

Square doesn’t release many details about its business. Zamost said more than four million merchants use its system to process “tens of billions” in gross merchandise volume annually. According to an internal projection shared last year with The Wall Street Journal, Square’s sales may reach nearly $1 billion this year, nearly double 2013’s revenue.